This invention relates to the field of measuring devices. More particularly, it relates to circuits for controlling display gauges, such as those utilized for displaying engine speed (tachometers), ground speed (speedometers), and distance travelled (odometers). Typically, such gauges are required on motor vehicles including passenger cars, trucks and tractors. Such gauges are important, particularly for trucks and tractors, where lease rates, service intervals and the like are dependent upon mileage while tractors must precisely monitor their ground speed to assure accurate placement of seeds during plowing.
Such devices, of course, are old in the art and have been primarily of the mechanical variety. For example, speedometers employ a cable attached to the wheel hub and the rotation of the cable is indicative of speed when a proper calibration is effected. More recently, electromechanical and electronic meters have come into use whereby an improvement in accuracy and dependability can be obtained. One such system is disclosed in U.S. Pat. No. 3,946,311 to Baker et al, assigned to the present assignee and hereby incorporated by reference. That patent discloses an air core gauge of the magnetic type in which the display gauge is driven by energizing a pair of electromagnetic coils for moving a pointer around a calibrated dial. The amount of angular displacement of the pointer is controlled by the amount of current flowing through the magnetic coils which are positioned at 90.degree. relative to each other. That patent discloses a method of obtaining an expanded pointer range by utilizing an electronic pulsing circuit for providing a time varying current flow through the magnetic coils in a manner so as to permit movement of the pointer through substantially 270.degree. of arc.
The present invention, as will be indicated in the descriptive portion of this specification, employs the pulse generating circuit disclosed in the referenced patent. The present invention, however, further discloses the concurrent operation of an odometer from the same circuitry and the capability for providing a user calibrated system.
In the past a substantial problem has existed with trucks and tractors when the gearing was changed as, for example, when a truck is in overdrive or when the tire size of a vehicle changes. Electronic measuring devices typically receive their input signals as a series of pulses from the wheel or drive shaft depending upon the condition being sensed. Changing the gear ratio of the final drive of a truck or changing the tire size on the wheels of the vehicle alters the calibration of the speedometer and odometer rendering them inaccurate until re-calibrated. Presently, regardless of the type of instrument employed, re-calibration has been an expensive and time consuming process whereby a skilled mechanic is required to adjust the device for the new operating parameters of tire size or gear ratio. Further, with respect to the overdrive gears present on many truck transmissions, it has been completely impossible to compensate for the use of a gear ratio of this type since it is employed only at highway speeds when constant driving conditions are likely to be encountered.
The present invention provides, in the several embodiments, the capability of solving these problems in an elegant manner. With respect to the re-calibration necessary for tire size changes and the like, a programmable divider is included in the pulse generating circuitry which can be programmed simply by inserting a properly coded key element into a receptacle. The coded key selects the dividing ratio of the programmable divider to account for changes in the condition to be measured. With respect to the truck overdrive problem, a simple circuit is provided which detects the occurrence of overdrive and switches the divider between two preselected operating modes, each of which is accurate for a given set of conditions. Re-calibration of the speedometer is automatically effective, according to the invention, for re-calibrating the odometer circuitry.
In another embodiment of the invention a form of the circuit is disclosed suitable for use in passenger automobiles where no changes in the measured parameters need be anticipated. In that embodiment a programmable divider is unnecessary and the pulse generating circuit is utilized for simultaneously driving the speedometer or tachometer and an associated odometer.
It is accordingly an object of the present invention to provide a user calibrated circuit for measuring a remote parameter, such as speed.
It is another object of the present invention to provide an electronic control circuit for driving a speedometer and odometer which circuit can be readily re-calibrated.
It is a further object of the present invention to provide an electronic control circuit which includes a programmable divider which can be set by a user to a selected value depending upon the application.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.